Comparing Energy, Mass, Speed, Wavelength, and Momentum of Photons and Electrons

[WeeBey]

Homework Statement

Compare a 2.2eV photon with a 2.2 eV electron in terms of energy, rest mass, speed, wavelength, and momentum

The attempt at a solution

So...

E = (2.2eV) x (1.60 x 10^-19 J/eV) = 3.52 x 10^-19 J

Wouldn't 3.52 x 10^-19 J be the energy for both the photon and electron? If so, wouldn't that make mass, speed, wavelength, and momentum equal for both particles?

 

[DaveC426913]

Well, hopefully it wouldn't make mass the same...

 

[gneill]

Tabulate all the values for each particle. What's the rest mass of a photon?

 

[WeeBey]

Isn't it:

m = E / c²?

Or is the rest mass of a photon always zero.

 

[gneill]

Isn't it:

m = E / c²


?

Or is the rest mass of a photon always zero.

The rest mass of a photon is always zero.

 

[WeeBey]

Okay, so the rest mass for the photon is 0 and for the electron it is 9.11 x 10^-31.

Wavelength would be:

E = hc / λ, or rather, λ = E / hc

Momentum would be:

p = h / λ

I assume because their energies are the same, the results for wavelength and momentum will be equal for the electron and photon.Speed is:

p = mv, or rather, v = p / m
In the end: their energies, wavelengths, and momentum are equal while mass and speed are different. Is that right?

 

[gneill]

Wavelength and momentum will not be the same. Look up DeBroglie wavelength, photon momentum.

 

[WeeBey]

Hmm,


So I first calculate velocity with:

v = √2eΔV/m

And then use:

λ = h / mv

That gives wavelength


For momentum I use:

p = mv


Because their masses are different, I should get different results.

So only the energy is the same for both?

 

[gneill]

As I said, you should tabulate all the values for each particle. You will have to be sure to use the appropriate formulas that apply to each. Then compare results.